The first draining of the Emosson dam since filling in 1973 took place in winter 2012 in order to inspect submerged parts of the dam and to undertake a necessary overhaul of the security equipment at the facility.

• Servicing of two head valves
• Servicing of two bottom drain valves
• Maintenance of several other pieces of equipment in the facility

The cost of all the works increased to around CHF 20m.

Some preparatory work had to be conducted in the preceding years such as bathymetry measurements of the lake to understand the topography of the lake floor.

The draining of the Emosson dam and works conducted necessitated the complete shutdown of the facility for 90 days during the winter of 2012. And the entire Emosson workforce was mobilised.

Many precautionary measures were taken in order to minimise the environmental impacts to the fullest extent possible, such as :

• Visiting the Eau Noire, Trient and Barberine rivers
• Diluting concentrations of sediments and adding clear water
• Multiple waterway flushes during and after emptying
• Drainage flow limited to 4m³/s

Despite all these measures, the death of almost all the fish could not be avoided due to lack of access to save them. In collaboration with the cantonal hunting and fishing service, a restocking measure was financed to the tune of CHF 100,000 by Electricté d’Emosson SA. The restocking of fishes took between 2 and 3 years depending on species.

Despite extreme meteorological conditions (heavy winter snow) and working at high altitudes (almost 2,000m), all maintenance actions went smoothly and the total emptying of the lake was a success. It is not envisaged for such an operation to be repeated until the time the concession ends in 2055.